I'm about to do a Comp test on my 94, I've never done one before so what I'd like to know is do I need to remove ALL the plugs before I start Or can I just test the cylinder/bank I suspect is faulty? or do I have to test all the cylinders,which I suppose is the sensible thing to do really.

 

I would want to know all.

 

Test them all... But you can test just one cylinder at a time without removing the other spark plugs...WW

 

Considering what a PITA it is to pull the #8 plug, you could be excused for not bothering, if you don't suspect a problem. You won't be breaking any cardinal rules by testing some, but not all cylinders.

Live well,

SJW

 

but #8 has the thinest part of the gasket and most likely to leak

 

It's easier on the battery if you take all the plugs out. Otherwise it doesn't matter. Remember to open the throttle fully to do the test to get maximum air into the engine.

 

And, if you don't pull all of the plugs, remember to disable the ignition.

Live well,

SJW

 

Others have posted good info. Just thought u should know what to look at/for. All cyl should be within 10% but that doesnt mean the engine wont drive and run with 1 low cyl. FWIW i had cyl #8 read like 140psi while all others read 180-190psi. Car ran fine and put another 10k-20k miles before changing engine. But something u should do if a cyl(s) read lower than others is use a squirt of oil and check compression again. If compression recovers it usually means worn rings in that cyl. If compression stays low it means the vlvs are leaking. Also the number of revolutions to get to a steady pressure will tell u how bad/worn the leak is. Another test would be a leak down test but this requires an air compressor. But the leakdown test is a good indicator of wear of each cyl.

One more thing is the engine should be warm for this as a cold engine wont seal as well as a warm one and your compression press will be lower. But the problem is how do u get the spark plugs out with the exh manifolds hot? Well u dont but most do the best they can to remove the plugs with the engine somewhat warm or at least not cold.

Hope this helps.